If you look closely on the backside/underside of the deck lid for the 400's there were cutouts for the emblem. I think this is what people are talking about being the difference. You wouldn't know the difference on a closed trunk lid if the 400 emblem were installed in the correct location.
1968 400 Coupe, verdoro green, black vinyl top, auto 1969 GTO Judge coupe, Carousel Red, manual 1971 Trans Am, cameo white, auto 1970 Buick Skylark Custom Convertible 350-4(driver), Fire Red, white top, auto 1972 Buick GS Stage 1, Royal Blue, black vinyl top, auto
